1. Field of the Invention
The present invention relates to a method for manufacturing a thin film magnetic head and particularly relates to a method for manufacturing a thin film magnetic head having a current perpendicular to the plane (CPP) type magnetoresistance effect element.
2. Description of the Related Art
As the reproducing element of a thin film magnetic head, a giant magneto-resistance (GMR) element is widely known. Although a current in plane (CIP)-GMR element is mainly used conventionally that allows a sense current to flow in the direction parallel to the film surface of an element, elements having a CPP structure that allow a sense current to flow in the direction orthogonal to the film surface of the elements have lately been developed in order to meet higher recording density. As this type of element, a CPP-GMR element that is the abovementioned GMR element having a CPP structure and a tunnel magneto-resistance (TMR) element using the TMR effect are known.
A CPP type element conventionally has a layered body (MR layered body, or MR stack) that constitutes a magnetoresistance film and has a magnetic layer (free layer) whose magnetization direction changes in response to an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is pinned relative to an external magnetic field and a nonmagnetic intermediate layer that is sandwiched between the pinned layer and the free layer. Moreover, both ends of an MR layered body in the layering direction is magnetically shielded by shield layers that also serve as electrodes for supplying a sense current to the MR layered body. The free layer is made to have a single domain by a bias magnetic field applied from bias magnetic layers, for example, provided on both sides of an MR layered body in the track width direction. As a result, the linearity of resistance change in response to the change in an external magnetic field can not only be enhanced, but Barkhausen noises can also be suppressed effectively. A relative angle between the magnetization direction of the free layer and the magnetization direction of the pinned layer changes in response to an external magnetic field. As a result, the electric resistance of a sense current that flows in the direction orthogonal to the film surface of a layered body changes. An external magnetic field can be detected by using this property.
In such a CPP type reproducing head, the base substrate including an MR layered body is placed in the air before a shield layer is formed on the upper portion of the MR layered body in the layering direction. At the time, a cap layer is provided on the upper surface of the MR layered body as a protecting layer in order to prevent the characteristics of the element from deteriorating due to the oxidation of the upper surface of the MR layered body. The cap layer is generally constituted of conductive material that is so-called heavy metal in view of process durability. The oxidized film formed on the surface of the cap layer by the abovementioned exposure to the air results in contact resistance between the cap layer and the upper shield layer, which leads to the deterioration of the characteristics of an element. For this reason, the surface of the cap layer needs to be removed before forming the shield layer. For the removal of the cap layer, dry etching such as ion etching and a reactive ion etching (RIE) is generally used. However, at the time of removing the cap layer made of heavy metal by etching, chemical elements of the cap layer are thrown into the MR layered body by a physical collision of ions (or ion implanting) at the time of etching, which inevitably causes damage to an element.
In order to decrease the influence of dry etching, Japanese laid-open application No. 2005-032780 proposes the structure of a cap layer that allows mitigating the damage to an MR layered body by shortening time required for removing the cap layer by dry etching. In this disclosure, the cap layer is constituted of two or more conductive layers; of those two or more conductive layers, a conductive layer on the shield layer side is constituted of a first material that is hardly oxidized or has small resistance if oxidized; and at least one remaining conductive layers are constituted of a second material containing chemical elements that are relatively high in oxygen binding energy compared with chemical elements constituting the major magnetic layer of the MR layered layer. As a result, the first material provided on the surface side of the cap layer allows decreasing the thickness of an oxidized film formed on the surface of the cap layer, and the second material provided on the MR layered body side allows decreasing the oxidation of the magnetic layer of the MR layered body, resulting in a rise in the MR ratio. It is therefore possible to reduce the deterioration of the characteristics of the MR layered body by dry etching by means of the first material that allows shortening the time required for the etching step and the second material that contributes to a rise in the MR ratio.
There is a need for a cap layer as thin as possible for higher linear recording density, that is, for the reduction of a gap between upper and lower shield layers (i.e., a read gap). In the cap layer having the abovementioned structure, however, the MR layered body may significantly be damaged by chemical elements in the cap layer that are thrown into the MR layered body if dry etching is conducted for a long time in order to sharply reduce the thickness of the cap layer. In addition, it is difficult to control the stop position of dry etching in the case of removing the entire cap layer, which may cause damage to the MR layered body due to over-etching or variation in the characteristics of the MR layered body due to the variation in stop position.